1. Field of the Invention
This invention relates to an improved dynamically vulcanized alloy ("DVA") which comprises a polyolefin thermoplastic resin, an elastomeric component, and an ethylene copolymer.
2. Description of Information Disclosures
Polymer blends which have a combination of both elastic and thermoplastic properties are of significant commercial interest. Such thermoplastic elastomeric compositions are generally obtained by blending an elastomeric polymer with a thermoplastic resin in a way such that the elastomer is intimately and uniformly dispersed as a discrete or co-continuous phase within a continuous or co-continuous phase of the thermoplastic polymer. These polymer blends have been given the generic designation of Thermoplastic Olefins ("TPO"). They exhibit some of the properties of a cured elastomer as well as the reprocessability of a thermoplastic resin. The elastomeric characteristics are enhanced if one component of the blend is a vulcanizable elastomer which is wholly or partially cross-linked.
The earliest work in the curing of a TPO composition was by Gessler and Haslett; see U.S. Pat. No. 3,037,954. That patent teaches the concept of "dynamic curing" wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer cured while continuously mixing and shearing the polymer blend. The resulting blend is a micro-gel dispersion of cured elastomer in an uncured matrix of resinous thermoplastic polymer.
Gessler's U.S. Pat. No. 3,037,954 discloses compositions comprising polypropylene and a rubber such as, for example, butyl rubber, chlorinated butyl rubber, polybutadiene, polychloroprene and polyisobutene. Compositions of about 50 to 95 parts polypropylene and about 5 to 50 parts of rubber are disclosed.
U.S. Pat. Nos. 3,758,643 and 3,806,558 disclose TPO type polymer blends comprising an olefin thermoplastic resin and olefin copolymer rubber wherein the rubber is dynamically cured to a partial cure state utilizing a curative such as a peroxide.
Since these early beginnings, numerous developments have taken place in the art of dynamically vulcanized alloys. These developments were generally directed towards the improvement and customizing of DVAs for specific uses. In particular, attention has focused upon the need for a soft, low Shore A hardness dynamically vulcanized alloy (DVA) having good flow properties and surface appearance while maintaining tensile strength, flexibility and elasticity. While an increase in elastomer content generally leads to a softer DVA, it also has a deleterious effect upon surface appearance. Generally when polyethylene or polypropylene is the thermoplastic resin in the blend, a high rubber content in the blend causes degradation of surface appearance. The proportion of rubber at which degradation of surface appearance occurs depends on the polymers used and the presence or absence of other compounding ingredients such as plasticizers and fillers.
U.S. Pat. No. 4,104,210 discloses dynamically vulcanized TPO blends which comprise about 25 to 45 parts of a polyolefin resin and from about 55 to 75 parts of a diene rubber which is a homopolymer or copolymer of a diolefin. U.S. Pat. No. 4,130,534 discloses dynamically vulcanized TPO blends which comprise up to 60 weight percent of a crystalline polyolefin resin and 40 weight percent or more of a butyl rubber. The butyl rubbers utilized include butyl rubber, and chlorinated or brominated butyl rubber.
TPO systems wherein the rubber is fully cured have the disadvantage that, as a result of poor flow characteristics, injection molded components made from such TPO's exhibit "flow line" imperfections. As a consequence, special mold designs are required to minimize the problem, especially for large parts. Uses are further limited because of the high Shore A hardness of such compositions.
U.S. Pat. No. 3,081,279 discloses compositions wherein a rubber, such as butyl rubber, is combined with a sulfochlorinated polyolefin resin and N-methyl-N-nitroso-P-nitroso-aniline and cured in a mold.
U.S. Pat. No. 2,369,471 discloses blends of ethylene polymers with various hydrocarbon rubbers and halogenated rubbers. The compositions are either uncured or compression molded vulcanizates.
U.S. Pat. No. 3,326,833 discloses compositions of a copolymer of ethylene with an alkyl ester of an alpha-beta-monoethylenically unsaturated monocarboxylic acid, e.g., ethylene-ethylacrylate copolymer, and a halogenated olefin polymer. The rubber can be halogenated butyl rubber or chlorinated polyethylene. The compositions when uncured are thermoplastic and when peroxide cross-linked form insoluble non-thermoplastic resins.
U.S. Pat. No. 4,639,487 discloses a TPO blend which utilizes as the thermoplastic continuous phase an ethylene vinyl ester or ethylene-alkylacrylate copolymer with a rubber component which may be a butyl rubber, halogenated butyl rubber, ethylene-propylene-diene rubber (EPDM), polyisoprene, polychloroprene, styrene-butadiene rubber, nitrile rubber, chlorosulfonated polyethylene, etc., wherein the rubber component is dynamically vulcanized to disperse the rubber as fully cured fine particles of a "micro-gel" within the thermoplastic matrix. Such TPO compositions possess exceptional resiliency, high coefficient of friction surfaces and low compression set. Although the TPO blends of the '487 patent have improved properties when such blends contain more than about 20 weight percent vinyl ester they tend to become cured or thermoset upon prolonged exposure to injection processing temperatures. Further, such compounds lack resistance to compression set at temperatures of the order of 100.degree. C. due to the low softening point of the ethylene copolymer resins.
U.S. Pat. No. 4,210,579 discloses a TPO blend including partially crystalline ethylene-propylene copolymer, crystalline polypropylene and 5-30 weight percent ethylene vinyl acetate (EVA) copolymer. No vulcanizing or crosslinking agents are used and all components are mixed together in a batchwise mode under heat and shear stress.
U.S. Pat. No. 4,607,074 discloses TPO compositions including at least two types of rubber and a thermoplastic resin, which may be EVA copolymer. At least one of the rubbers is dynamically vulcanized to a fully vulcanized state by a curing agent which does not vulcanize the other rubber.
U.S. Pat. No. 4,593,063 discloses TPO blends containing up to 35 weight percent of elastomer while maintaining high gloss surface appearance.
U.S. Pat. No. 4,894,408 discloses thermoplastic compositions comprising an ethylene copolymer resin and a dynamically vulcanized fully cured EPDM rubber. The ethylene copolymer resin may be a copolymer of ethylene with vinyl acetate, copolymers of ethylene with alpha, beta monoethylenically unsaturated monocarboxylic acids and copolymers of ethylene with alkyl esters of said acids.
Attempts to produce a softer, lower durometer TPO blend product having good surface appearance and flow properties have concentrated upon increasing the amount of rubber in the blend. While increasing rubber content generally leads to a softer product, if the rubber is fully cured, then the surface appearance, the flow properties and the softness of the product are adversely affected.
It has now been found that the addition of certain copolymers of ethylene to the thermoplastic composition after the dynamic vulcanization of the specified elastomers and a polyolefin-olefin resin improves the surface appearance of softer thermoplastics.